There appears to be a transport barrier for oxygen between the red blood cell hemoglobin and the myocyte: intracellular oxygen tensions are lower than those in the venous outflow, and indicators dilution curves for O-18 labeled oxygen give indication of barrier limitation. In the proposed study data obtained with both O-18 and O-15 will be examined with respect to the question of the location of the barrier or the relative permeability-surface area products of the succession of barriers lying between hemoglobin and mitochondrial enzymes. The O-15 oxygen can still be measured after transformation to O-15 water, while O-18 water is indistinguishable from background. The combining of information from the two labels maximizes the possibility of a correct and complete analysis. The specific aims are as follows: 1. to acquire and analyze data from the in situ working heart of dogs using O-18 oxygen; and 2. to acquire and analyze data in similar preparations using O-15 positron emission tomography and outflow dilution simultaneously, using microsphere depositions to measure local myocardial blood flow.